Link utilization control mechanism for demand assignment satellite communications network

ABSTRACT

A link utilization control mechanism for a demand assignment satellite communication network employs a modified point-to-point communications protocol (X.25) in order to simulate point-to-point communication ports and thereby interface what is effectively a multidrop network with point-to-point landlink communication resources. Through an acknowledgement reservation mechanism the return link channel is subdivided into interleaved data and (preassigned) acknowledgement time slots, in order to substantially obviate overhead and throughput penalties encountered in the use of large data packets to transmit reduced size acknowledgement messages. In addition, the normal contention mode of operation of the return link is augmented with a data time slot reservation mechanism through which, during periods of increased message input density at a remote station, transmission throughput is facilitated, so that congestion at the remote station is reduced.

This is a continuation of application Ser. No. 236,756, filed Sep. 26,1988, now U.S. Pat. No. 5,003,534, issued Mar. 26, 1991.

FIELD OF THE INVENTION

The present invention relates in general to communication systems and isparticularly directed to a link utilization control mechanism forcontrolling allocation and throughput of the data transmission links ofa satellite communications network.

BACKGROUND OF THE INVENTION

Digital data (packet switching) communication networks haveconventionally employed dedicated terrestrial circuits, such as landlinetelephone systems, to connect a host (mainframe) computer, located at acentral or master station, with a plurality of geographically dispersedremote terminals, the locations of which are selected in an effort tomeet current and projected communication demands of the system user.Because a dedicated landline telephone link in a multidrop network is aneffectively rigid physical communication highway and typically employssome form of master-to-remote polling (point-to-point) mechanism forcontrolling communications between the master station and the remotestations, both the number and the locations of the stations of thenetwork must be carefully chosen. In addition, it is common practice inpacket-switched landline transmission networks to use point-to-pointcommunication protocols between the user terminal and a network entrynode, which require dedicated channel connections between thecommunication ports of the packet switches of the respective stations.

A satellite communication network, on the other hand, offers the usersignificant flexibility in the deployment of the stations, but isnormally does not allow the use of a polling mechanism for controllingaccess to the communication channel, as in the case of a terrestrialsystem, because of the substantial transmission delay (wait) penaltythat would be incurred. Consequently, a satellite communications networkmay preferably employ a communication channel that is accessed on acontention or demand assignment basis by the stations, in order toafford maximum, efficient utilization. In such a network, communicationsfrom the master station to the remote stations (cutlink transmissions)are broadcast over a first, continuously transmitted frequency (channel)that is monitored by each remote station for messages addressed to it.Messages from the remote stations to the master station (return linktransmissions) are transmitted over a second, shared channel, in burstmode, contention format.

Because of the manner in which the satellite communications channels areshared among a plurality of stations, they cannot be readily interfacedwith terminal equipment (packet assembly/disassembly circuits forcoupling the satellite network to existing landline connection ports).Namely, the local packet interface equipment may typically employ apoint-to-point communication protocol, such as X.25 communicationprotocol, the station-to-station control layer of which contains atransmit/receive channel designation field and implies point-to-pointutilization, exclusively. In order for such a protocol to be usable in amultistation satellite network, each earth station (master or remotes)would require a separate channel and port dedicated to each terminalbeing serviced, something that is practically impossible to achieve in asystem that may serve thousands of terminal devices and, because of itsuse of a shared communications channel, effectively appears as amultidrop network, which is inherently incompatible with point-to-pointcommunication protocols.

An additional problem that is encountered in the use of a shared(contention) communications network is the need for acollision/avoidance mechanism on the shared (remote-to-master) link.Namely, although outlink messages from the master station to the remotestations originate at only a single source (the master station), so thatthe issue of master-to-remote transmission collisions does not exist,remote stations transmit over the return link channel on a contentionbasis, so that there is the possibility for remote-to-mastertransmission collisions.

Efforts to reduce the collision problem in networks employing sharedcommunication channels have included a variety of "permission"-basedcommunication protocols, such as polling mechanisms (intolerable in asatellite network, as noted previously) and time division multipleaccess transmission formats, which operate, in effect, like pollingmechanisms. In a commercial environment, where every effort is made tooptimize channel occupancy and throughput, the delay penalty of suchprotocols makes the unacceptable candidates for handling traffic thatmay originate from literally thousands of system users (terminaldevices) that are served by the stations of some networks.

Unfortunately, conventional collision avoidance/recovery schemes (suchas that used in a slotted Aloha communication control mechanism) areeffectively unworkable for the class of earth stations known as VSATs(very small aperture terminals) due to the fact that the transmitting(remote) stations are unable to monitor their own signals, because ofthe VSAT's small antenna and low transmit power. Instead, they rely onthe transmission of acknowledgements from the master station to confirmmessage throughput. Similarly, master-to-remote messages areacknowledged by the remote stations.

Because an acknowledgement is essentially overhead, in terms of messagesize, it's length is small (on the order of ten bytes or less) comparedwith the length of a normal data packet (on the order of a thousandbytes). As a consequence, its impact on channel efficiency isparticularly noticeable when this or other type of reduced contentoverhead messages is transmitted as a `partially-filled` data packetduring a normal, fixed data time slot, the remaining unused portion ofwhich may occupy a considerable percentage of the available transmissioninterval.

A further difficulty that is encountered in demand assignment, burstmode transmission schemes is the substantial reduction in networkthroughput that occurs when incoming (to be transmitted) traffic atremote stations build up to a level that effectively overloads thenetwork, or reaches the onset of a saturation condition, so as tosubstantially increase transmission delay to the point that nearly everypacket must be retransmitted, due to collisions with other bursts. As aresult, the likelihood of a message from a remote station successfullyreaching the master station is infinitesimally small, thus reducingnetwork throughput to zero.

SUMMARY OF THE INVENTION

Pursuant to a first aspect of the present invention there is provided acommunication interface mechanism that enables messages to betransmitted over a shared communications channel by means of apoint-to-point communications protocol, such as internationally employedX.25 protocol, so as to facilitate interfacing of the satellite networkwith to conventional landlink communication resources. In particular,within the satellite communications network, digital informationpacket-containing messages are conveyed between a first (master) stationand a plurality of remote station over respective dedicated channels (afirst, master-to-remote outlink broadcast frequency and a second,remote-to-master return link frequency). The master station contains apacket switch having one or more first ports into which outgoingmessages, such as data packets supplied by one or more host mainframecomputers for transmission to second terminal devices at the remotestations, are coupled, and from which incoming messages on the returnlink channel from the remote stations are output to the hostcomputer(s). The master station's packet switch also includes one ormore second ports through which outgoing messages it has received fromthe host computers are coupled to the outlink channel for broadcast toeach remote station and to which incoming messages received from thereturn link channel are applied. Within the packet assembly/disassemblydevice at each station, data packets are assembled for transmission bymeans of a point-to-point communication protocol, such as theabove-mentioned X.25 protocol, which is inherently incompatible with theshared communications channels of what is, in effect, a multi-dropsatellite network, rather than a point-to-point network for which thecommunications protocol is designed.

In accordance with the present invention, this inherent inconsistencybetween (X.25) point-to-point communication protocol and a multidropnetwork is obviated by a modification of the packet switch at eachstation and a modification of the station-to-station layer of theprotocol, so as to enable the outlink and return link channels toeffectively simulate point-to-point communications therebetween. Formessages transmitted from a remote station to the master station, themodification of the protocol comprises incorporating into each messagean auxiliary identification code (such as an additional (abbreviated)two byte, subaddress field) which identifies the remote station sourcingthe message. At the master station, the packet switch is provided withan auxiliary memory space, containing a plurality of pseudo port entries(queues), into respective ones of which return link messages coupledfrom an attendant satellite communications modem to a second port of thepacket switch, are stored or buffered, and the addresses of which aredesignated in accordance with the identification codes of the remotestations contained within the received messages. The master station'spacket switch outputs each buffered (X.25) point-to-point protocolmessage, absent its auxiliary identification code, via a first port ofthe packet switch to its associated packet assembly/disassembly device,so that the data may be forwarded to its destination host computer.Thus, to the packet assembly/disassembly device, which interfaces theuser equipment and the packet switch, communications appear to beeffected through dedicated ports of its associated packet switch to apoint-to-point link to the remote station.

Consistent with the modification of point-to-point protocol forremote-to-master station communications over the contention return linkchannel, outgoing messages from a host computer, and coupled from themaster station packet assembly/disassembly device to a first port of thepacket switch for transmission to a remote station, are initiallybuffered in the pseudo port entry of the auxiliary memory space of themaster station's packet switch, whose address corresponds to theidentification of the destination remote station and which appears topacket assembly/disassembly device as a dedicated packet switch outputport having a point-to-point connection to the remote station. In thecourse of outputting the buffered message via a second port forapplication (by its attendant modem) to the master-to-remote channel,the master station's packet switch incorporates into that message theauxiliary two byte address (the pseudo port entry where the bufferedmessage is stored) which identifies the destination remote station. Themessage is then broadcast by the master station's modem over the outlinkchannel to each of the remote stations.

The satellite communication modem at each remote station continuouslymonitors the master-to-remote channel for messages that may be addressedto it, namely, for the presence of its own identification code withineach message broadcast by the master station. When a remote stationdetects its identification code, it captures the message and thenoutputs it on to its associated packet assembly/disassembly device,absent the station identification code, so that, to that destinationterminal device, it appears as though it has received a message from themaster station over a dedicated point-to-point link.

In accordance with a second aspect of the present invention, theoverhead and throughput penalties encountered in the use of data packetsto transmit small acknowledgement messages are obviated by a channelutilization mechanism that subdivides the availability of the returnlink channel into a first sequence of data fields or time slots, accessto which is normally acquired on a contention basis, and betweensuccessive ones of which a second sequence of reduced informationcapacity overhead time slots (acknowledgement frames) are interleavedfor use by the remote stations to transmit acknowledgements over thereturn link to the master station.

In particular, whenever the master station transmits an message to aremote station, it includes, as part of the message, the identificationof a prescribed acknowledgement time slot, relative to a reference timeoccurrence, within which an acknowledgement message is to be returned bythe remote station. (Acknowledgement packets contain sequence numberswhich identify the outlink message being acknowledged.) Each remotestation monitors the master-to-remote outlink channel for a messagetransmitted to it from the master station and, in response to receipt ofa message from the master station, transmits an acknowledgement messageback to the master station during a time slot as identified as part ofthe received message.

Because the length of an acknowledgement message (usually on the orderof ten bytes or less) is considerably shorter than the length of a datapacket (often up to one thousand bytes), the reserving of suchacknowledgement frames does not detrimentally impact channelutilization. Moreover, preassigning or reserving these reduced capacityslots for return-to-master acknowledgements serves to minimizecollisions and thereby improve overall network performance.

In the course of the control of assembly and transmission of a datapacket to a remote station, the communications control processor assignsto the recipient remote station a reserved acknowledgement time slotcode by referencing that acknowledgement time slot to a network timingsignal that is continuously modulated onto the outlink carrier. Thecommunications control processor within the master station also storesthe most recent acknowledgement time slot reservation code in a reservedacknowledgement table in order to assure uncontended use of theacknowledgement time slots. Then, as acknowledgements are returned fromthe remote stations the master station controller knows that it does nothave to retransmit the original packet. The underlying datacommunication protocol (e.g. X.25) includes a timer, so that in theevent that the acknowledgement is not returned within a prescribedperiod of time, the packet will be transmitted and a new acknowledgementslot assigned, thereby permitting the master station communicationscontroller to keep track of whether transmitted data packets have beenreceived and which packets have not been received and need to beretransmitted.

In the course of handling input messages from user equipment fortransmission over the return link channel, the message buffer within thecommunications control unit of the remote station queues data packetssupplied by its associated packet assembly/disassembly unit or PAD.Similarly, whenever the PAD has successfully received a data packet fromthe master station, the communications processor extracts the includedacknowledgement slot reservation and stores the reservation in a list ininternal memory. The data packet is then passed to the PAD.Subsequently, the PAD may generate one or more acknowledgements (ornegative responses).

Pursuant to a redundancy elimination mechanism in accordance with thepresent invention, as acknowledgements arrive at the communicationsprocessor, they are placed in a first-in/first-out (FIFO)acknowledgement reservation buffer, if there is an upcomingacknowledgement reservation in the list. If, for some reason, there areno upcoming acknowledgement reservations in the list, theacknowledgement packets are placed in a separate data FIFO buffer andtreated as data packets for the purpose of transmission. As anacknowledgement packet is about to be placed in either the data FIFObuffer or the acknowledgement FIFO, its contents are are examined todetermine if the new acknowledgement contains more currentacknowledgement information than those currently buffered and awaitingtransmission. (It should be noted that an acknowledgement of a packetimplicitly acknowledges any previous packets.) If so, the acknowledgmentcontents are replaced with the new information. Thus, acknowledgementtraffic is kept to a minimum by eliminating redundant packets.

As each acknowledgement or data slot occurs, the communicationsprocessor decides whether or not to transmit into the slot. If both theacknowledgement reservation list and acknowledgement FIFO are not empty,the communications processor withholds all transmissions until theacknowledgement slot occurs. At that time, the acknowledgement packet istransmitted into the reserved acknowledgement slot and normal processingresumes. This procedure insures proper sequencing of data andacknowledgement packets.

It should be noted that the PAD, upon transmitting a data packet to themaster station, typically will repetitively generate a `poll` packet forsome period of time until it receives an acknowledgement packet from themaster station. These repeated `poll` packets are a potential source ofmessage traffic congestion, but are still treated as normal messagepackets by the communications control unit. Consequently, the use of theacknowledgement redundancy mechanism serves to eliminate superfluoustransmissions over the return link channel.

Pursuant to still another feature of the invention, during periods ofincreased message input density at a remote station, resulting in anincreased incidence of collisions on the return link channel and theneed to retransmit multiple data packets that are awaiting service(queued) at a remote station, transmission throughput is facilitated byan adaptive data slot reservation mechanism that responds to the hightraffic density condition and reserves or assigns data time-slots foruse by that remote station, so that potential congestion at the stationis reduced.

Namely, as pointed out above, in the course of handling input messagesfor transmission on the return channel to the master station, the remotestation transmission buffer queues up data packets supplied by thepacket assembly/disassembly unit through which the remote stationinterfaces with terminal communication links (e.g. a terrestrial localarea network telephone system) that supply messages from user terminalsto be transmitted to the master station and for whom received messagesare to be delivered. Pursuant to this additional aspect of theinvention, the contents of the return link transmission buffer aremonitored. In response to the occurrence of a prescribed condition ofthe contents of the buffer, specifically a condition in which the numberof messages awaiting transmission has reached a preselected number andthe buffer contains a message that has been previously transmitted andis awaiting retransmission, the remote station interrupts normaloutputting of queued packets from the transmission buffer and outputsinstead only the leading buffer, tagged with a request for thereservation of contention time slots to be used for the transmission ofall the remaining data packets currently awaiting service in the queue.

Upon receipt of a message from the master station containing theidentification of contention time slots that are to be reserved for useby the requesting remote station, the remote station proceeds totransmit messages stored in its transmission buffer over the returnchannel to the master station during the reserved time slots. If therequesting remote station does not receive a reservation message fromthe master station within a prescribed period of time after transmittingthe reservation request, it retransmits the request several times andfailing that it proceeds to transmit messages stored in the transmissionbuffer over the return link channel during non-reserved contention timeslots.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a satellite communicationssystem employing the improved link utilization control mechanism inaccordance with the present invention;

FIG. 2 diagrammatically illustrates a modification of packet switchmemory space for providing a plurality of `pseudo` ports through whichpoint-to-point connections may be simulated;

FIG. 3 shows a modification of X.25 protocol in which a subaddress fieldSUBADDR is appended to the address field;

FIG. 4 diagrammatically illustrates the manner in which acknowledgementtime slots and data packet time slots are interleaved with one another;

FIG. 5 diagrammatically shows a transmission buffer in which data andacknowledgement packets are queued; and

FIG. 6 is a state diagram of a control mechanism employed for requestingreservation of data time slots;

DETAILED DESCRIPTION

Before describing in detail the particular improved link utilizationcontrol mechanism in accordance with the present invention, it should beobserved that the present invention resides primarily in a novelstructural combination of convention communication and signal processingcircuits and components and not in the particular detailedconfigurations thereof. Accordingly, the structure, control andarrangement of these conventional circuits and components have beenillustrated in the drawings by readily understandable block diagramswhich show only those specific details that are pertinent to the presentinvention, so as not to obscure the disclosure with structural detailswhich will be readily apparent to those skilled in the art having thebenefit of the description herein. Thus, the block diagram illustrationsof the Figures do not necessarily represent the mechanical structuralarrangement of the exemplary system, but are primarily intended toillustrate the major structural components of the system in a convenientfunctional grouping, whereby the present invention may be more readilyunderstood.

Referring now to FIG. 1, a diagrammatic illustration of a satellitecommunications system employing the improved communication controlsystem in accordance with the present invention is illustrated ascomprising a master station 10 which communicates via a satellite 20with each of a plurality of remote stations 30, so that, in effect, thesatellite communications network may be considered as what is normallyreferred to as a star-configured network, with the hub of the starcorresponding to master station 10 and the points of the starcorresponding to the remote stations 30. Master station 10 broadcastsmessages on a first continuously transmitted (Ku band) outlink carrierthrough satellite 20 to all of the remote stations 30. Each of remotestations 30 continuously monitors the outlink channel for messagesdirected to it, as identified by the contents of a remote stationaddress contained within the station-to-station layer of thecommunication protocol, as will be described below. Each remote station30 transmits message to master station 10 in a burst-mode format,through satellite 20 over a dedicated remote-to-master (Ku band) returnlink channel.

The master station 10 itself is shown as comprising a satellitecommunication antenna 11 for transmitting and receiving Ku band signalsvia satellite 20 by way of associated master data communicationsequipment (MDCE) 13. Master data communications equipment 13 includes aRF transceiver unit 21, a received carrier output port 21R of which iscoupled to a return link burst demodulator 23 and an outlink continuousmodulator input port 21T of which is coupled to a continuous modulator25. The respective data communication ports 23D and 25D of burstdemodulator 23 and continuous modulator 25 are coupled to a packetswitch 27, through which transmitted and received messages areinterfaced to an associated packet assembly/disassembly device (PAD) 28,for coupling data packets with respect to one or more host terminaldevices (such as mainframe computers) 40, serviced by the MDCE 13. Eachof the burst continuous demodulator 25, packet switch 27 and PAD 28 arecontrolled by an attendant master station communications controlprocessor 29.

Each remote station 30 is configured similar to the master station 10,in that it includes a satellite communications antenna (such as a verysmall aperture terminal (VSAT) antenna dish) 31, coupled with an RFtransceiver unit 41 of associated remote data communications equipment(RDCE) 33. Within RDCE 33, the RF transceiver unit 41 is coupled to acontinuous carrier demodulator 42 for demodulating incoming messagesfrom master station 10 and an outgoing burst modulator 43 for effectingreturn link carrier burst communications of messages that originate atthe remote station 30 for transmission to master station 10. Each ofcontinuous demodulator 42 and burst modulator 43 is coupled to acommunications control unit 45 which includes a communications controlprocessor 46 and a packet assembly/disassembly device (PAD) 48, forcoupling data packets with respect to one or more user terminal devices50, serviced by the MDCE 13. Each of continuous demodulator 42, burstmodulator 43 and PAD 48 are controlled by an associated remote stationcommunications control processor 46.

Except for the communication control mechanisms, to be described belowwith reference to FIGS. 2-6), employed by communications controlprocessor 29 and associated packet switch 27 within the master datacommunications equipment 13 at master station 10 and that employed bycommunications control processor 46 at the remote station 30, theconfiguration and hardware components employed by each of the masterstation 10 and the remote station 30, described above, are conventionaland will not be described in detail here. Rather, the description tofollow will address the details of the modifications to the packetswitches and the communications protocol, summarized briefly above,through which the link utilization control mechanism of the presentinvention is implemented.

Before describing the details of the present invention, it is useful tobriefly review the operation a star-configured satellite communicationnetwork employing continuous mode outlink transmissions from the masterstation to the remote stations and burst mode return link transmissionsfrom the remote stations to the master station.

At the master station 10, master data communications equipment 13 isported to one or more host terminal devices 40 that may source orreceive digital data communications. When a host device 40 desires tocommunicate with user equipment 50 that is serviced by a remote station30, it forwards a data communications request and any attendant dataover its local link to a host port 28HP of PAD 28. PAD 28 takes eachtransmission request, assembles the necessary outlink message packet(s)and then forwards the packet to packet switch 27, wherein the message istemporarily buffered for application to continuous carrier modulator 25and transmission via RF transceiver unit 21 and antenna 11 over theoutlink satellite channel. The formatting, assembly and disassembly ofmessages is controlled within master data communications equipment 13 bycommunications control processor 29, which contains the link utilizationcontrol mechanism of the present invention to be described in detailbelow.

In order to provide a synchronization reference for all of the users ofthe network, master station 10 modulates continuously transmittedoutlink carrier with a time slot marker, which is monitored by eachremote station to regulate when return link data message time slots andacknowledgement message time slots (which are reserved or preassigned bythe master station) occur. As pointed out briefly above, and is will beexplained in detail below, return link data message time slots arenormally accessed by the remote stations on a contention basis, but maybe reserved by the master station in response to request by a remotestation that has encountered a transmission congestion condition. Alsomodulated onto the continuous carrier are packets output from packetswitch 27 to continuous modulator 25 for broadcast on the outlinkchannel in response to requests from host terminal devices 40. Incomingburst-mode return link messages from the remote stations are demodulatedby burst demodulator 23 are buffered and disassembled through packetswitch 27 and PAD 28 and then output to a destination host terminaldevice 40.

At each remote station 30, the outlink (master-to-remote) channel ismonitored continuously for any message that may be addressed to thatparticular remote station by the master station 10. Namely,communications control unit 45 continuously monitors the output ofcontinuous demodulator 42 for the presence of messages that contain theaddress of that remote station. When the remote station sees its ownaddress, it then captures the message and buffers its contents withinPAD 46, so that it may be disassembled and output therefrom to userequipment 50 over a local communication link that connects the userequipment to the PAD. Similarly, outgoing messages from user equipment60 (such as data to be transmitted in response to a file request from ahost computer 40 at the master station) are coupled to PAD 46, which theassembles the packet(s) for application to burst modulator 43 andtransmission in burst format on the return link channel through thesatellite.

As pointed out briefly above, a communication protocol often used forterrestrial packet switching data transmission systems, particularlyland-link telephone networks employing terminal-to-terminal andterminal-to-host communications, is X.25 communication protocol, thestation-to-station control layer of which contains a transmit/receivechannel designation (address) field and implies point-to-pointutilization, exclusively. Normally, such a protocol is not usable in ashared channel multi-station satellite communication network, sincepoint-to-point communications require dedicated links between eachstation.

In a shared link network configuration as shown in FIG. 1, the protocolemployed for station-to-station communications is normally designed tooperate with a limited number of user interfaces, which inevitablyresults in a higher subscription cost to the user and often makes accessto a satellite communications network prohibitively expensive.

Pursuant to a first aspect of the present invention there is provided acommunication interface mechanism that enables messages to betransmitted over the shared satellite communications channel by means ofa modified version of the above-referenced X.25 point-to-pointcommunications protocol, which facilitates interfacing of the packetswitches of each station of the network with its associated packetassembly/disassembly device and enables the outlink and return linkchannels to effectively simulate point-to-point communications betweenthe master and remote stations.

For messages transmitted from a remote station 30 to master station 10,the modification of the protocol comprises incorporating into eachmessage an auxiliary identification code (such as an additional(abbreviated) two byte, subaddress field) which identifies the remotestation sourcing the message. At the master station 10, packet switch 27is provided with an auxiliary memory space, containing a plurality ofpseudo port entries, into respective ones of which return link messages,coupled via one or more input ports 27 IDP from burst demodulator 23,are stored or buffered, and the addresses of which are designated inaccordance with the identification codes of the remote stationscontained within the received messages. Packet switch 27 outputs eachbuffered (X.25) point-to-point protocol message, absent its auxiliaryidentification code, via an output port 270HP to PAD 28, so that thedata may be forwarded to a destination host computer 40. Thus, to PAD28, which interfaces the host terminal device 40 with packet switch 27,communications appear to be effected through dedicated ports of packetswitch 27 to a point-to-point link to a remote station 30.

Consistent with the modification of point-to-point protocol forremote-to-master station communications over the contention return linkchannel, outgoing messages from a host computer 40, and coupled from themaster station PAD 28 to an input port 271HP of packet switch 27 fortransmission to a remote station, are initially buffered in the pseudoport entry of the auxiliary memory space of the master station's packetswitch, whose address corresponds to the identification of thedestination remote station and which appears to PAD 28 as a dedicatedpacket switch output port having a point-to-point communication link tothe remote station. In the course of outputting the buffered message viaan output data port 270DP for application to continuous modulator 25 andtransmission over the outlink channel, packet switch 27 incorporatesinto that message the auxiliary two byte address (the pseudo port entrywhere the buffered message is stored) which identifies the destinationremote station. The message is then broadcast by the master station overthe outlink channel to each of the remote stations.

The communications control unit 45 at each remote station continuouslymonitors output of continuous demodulator 42 for messages that may beaddressed to it, namely for the presence of its own identification codewithin each message broadcast by master station 10. When a remotestation detects its identification code, it captures the message andthen outputs it on to its associated packet assembly/disassembly device,absent the station identification code, so that, to that destinationterminal device, it appears as though it has received a message from themaster station over a dedicated point-to-point link. Thus, to users ofthe network, it appears that communications are point-to-point, while,in reality, they are carried out over what is effectively a sharedmulti-drop network.

Referring now to FIG. 2, the above-referenced modification of the packetswitch memory space, so as to effectively provide a plurality of`pseudo` ports through which point-to-point connections may be simulatedfor the use of X.25 protocol, is diagrammatically illustrated as a table71 having a plurality of message entries 71-l....71-N, each of which isa queue that stores messages to be transferred to and from one of ports271DP and 270DP (to which the master station modem equipment(demodulator 23 and modulator 25) is coupled). The host ports 271HP and270HP are coupled to the modem ports 271DP and 270DP via the packetswitching unit 30. The address of each entry of `pseudo` port table 71specifies a `pseudo` port to which the modem 23/25 is connected, ratherthan an actual hardware port 271DP/270DP. Each `pseudo` port address isthe address of one of the remote stations 30 with which the masterstation 10 may communicate.

In conventional point-to-point communications protocol, such as theabove-mentioned X.25 protocol, the communications control layer whichdefines station-to-station transmissions includes an address frame whichsimply prescribes the outlink and return link channels (reversed foropposite ends of the link). Consequently, whenever a host device 40serviced by master station 10 desires to communicate with user equipment50 at a remote station 30, it forwards that message to PAD 28, whichassembles a data packet message using X.25 protocol and couples the datapacket message to packet switching unit 30 within packet switch 27 towhat packet switching unit 30 thinks is a hardware port dedicated topoint-to-point communications to the destination remote station. Inaccordance with the present invention, however, the message, in reality,is directed to that one of the `pseudo` port entries of table 71 whoseaddress is the identification of the destination remote station. Still,as far as packet switching unit 30 is concerned, the message is beingported to a dedicated communications link, compatible with the X.25protocol it is using.

In accordance with the modified point-to-point protocol controlmechanism of the present invention, the address of the accessed entry oftable 71 is used to define an additional (two byte) subaddress field(which identifies the remote station for whom the packet is intended),which is inserted into the communication layer through whichpoint-to-point communications using X.25 protocol are normally defined.This modification is illustrated in FIG. 3 which shows a typical X.25point-to-point message having front end and rear end flag bytes FLGbetween which address ADD, control CNTRL, data DATA and frame checksequence FCS fields are inserted. Pursuant to the present invention, theadditional two-byte subaddress SUBADDR is appended to the address fieldby the communications control processor as it forwards the contents of a`pseudo` port table entry 71-i to continuous modulator 25 by way ofpacket switch output port 270DP.

At the remote station 30, the subaddress field of each message packettransmitted from master station 10 output from continuous demodulator 42is examined by communications control processor 46 to determine whetherits subaddress field SUBADDR identifies that remote station. When thecommunications control processor 46 determines that the subaddress fieldidentifies that remote station, it causes the incoming message to becoupled to PAD 48, but removes the subaddress from the point-to-pointprotocol layer that was inserted in accordance with the operation of the`pseudo` port mechanism at the master station, described above. Themessage is then output over the local communications network to whichthe remote station is coupled for transmission to the intended userequipment 50.

Conversely, when a message packet is assembled by PAD 48 fortransmission over the return link channel to master station 10, theaddress of that sourcing remote station is inserted by itscommunications control processor 46 as the above-mentioned subaddressfield in the point-to-point channel definition layer of the X.25communications protocol. Then, at the master station, when the incomingburst message is coupled from burst demodulator 23 to input port 271DPof packet switch 27, control processor 29 uses the subaddress field todirect the incoming message to its corresponding entry in `pseudo` porttable 71 absent the subaddress field. Packet switching unit 30 thencouples the contents of that entry of the `pseudo` port table 71 to PAD28 via port 1HP for delivery to the destination host terminal device 40.

To each of the packet switching unit 30 (and, consequently, PAD 28 atmaster station 10) and PAD 48 at remote station 30, the additionaltwo-byte subaddress is effectively invisible, so that it appears todevice that there is a direct point-to-point connection between theremote station and the master station, rather than a sharedcommunication channel therebetween, so that the end user of the networkis able to use conventional X.25 protocol, as is, yet have access to ashared communications network.

In a communication network employing a shared communications channel, itis common practice to employ acknowledgement messages to confirm receiptof a data packet. Conventionally, sending an acknowledgement message hasinvolved sending a packet, the information contained within whichessentially indicates that the data packet of interest was successfullyreceived, so that the source station need no longer retain or store thatdata packet for retransmission (as would be necessary, for example, inthe case of a collision, the retransmission being governed by aprescribed collision recovery mechanism). A shortcoming in slottedchannels in sending acknowledgement packets in data slots is the factthat the acknowledgement message normally requires only a few bytes ofinformation, whereas a data slot is large enough to contain a datapacket of up to, usually, 128 to 1,000 bytes, depending on systemconfiguration. In other words, using data slots for overhead (e.g.acknowledgements) constitutes an extremely inefficient utilization ofthe satellite channel.

In accordance with the present invention, this waste of a preciousresource (the shared/contention return link channel) is obviated bysubdividing the time slots during which burst mode communications fromthe remote stations to the master station may take place intointerleaved sequences, one of which contains (relatively long duration)data packet time slots and the other of which is comprised of (veryshort duration) overhead time slots. Because the duration of eachacknowledgement (overhead) time slot is only a fraction of the portionof a data packet time slot, that channel occupation efficiency can beeffectively enhanced.

Each acknowledgement time slot on the return link channel is reserved orpreassigned by the master station when the master station transmits adata packet to a remote station, by including as part of the informationin the data packet the identification of a subsequently occurringoverhead time slot during which the remote station is to transmit itsacknowledgement of receipt of that data packet back to the masterstation.

The manner in which the acknowledgement time slots and data packet timeslots are interleaved with one another is diagrammatically illustratedin FIG. 4, which shows a sequence of data time slots D_(j-1), D_(j),D_(j+1), D_(j+2) and interleaved acknowledgements time slot Ak_(i-1),Ak_(i), Ak_(i+1). As noted previously, on the outlink channel, themaster station broadcasts a continuous carrier that is monitored by allof the remote stations of the network. Modulated onto this carrier is aclock signal upon which system timing for all users of the network isbased. All time slots, whether they be data time slots oracknowledgement time slots, are referenced to the network clock.

In the course of the control of assembly and transmission of a datapacket to a remote station, the communications control processor 29assigns to the recipient remote station a reserved acknowledgement timeslot code by referencing that acknowledgement time slot to the networktiming signal that is continuously modulated onto the outlink carrier.The communications control processor 29 within master station 10 alsostores the most recent acknowledgement time slot reservation code, inorder to assure uncontended use of the acknowledgement time slots. Then,as acknowledgements are returned from the remote stations, the masterstation controller knows that it does not have to retransmit theoriginal packet. The underlying data communication protocol (X.25 in thepresently described embodiment) includes a timer, so that in the eventthat the acknowledgement is not returned within a prescribed period oftime, the packet will be transmitted and a new acknowledgement slotassigned, thereby permitting the master station communicationscontroller to keep track of whether transmitted data packets have beenreceived and which packets have not been received and need to beretransmitted.

More particularly, in the course of handling input messages from userequipment 50 for transmission over the return link channel, the messagebuffer within communications control unit 45 of the remote station 30buffers or queues data packets supplied by the packetassembly/disassembly unit 48. Similarly, whenever PAD 48 hassuccessfully received a data packet from the master station,communications processor 46 extracts the included acknowledgement slotreservation and stores the reservation in a list in internal memory. Thedata packet is then passed to PAD 48.

Subsequently, PAD 48 may generate one or more acknowledgements (ornegative responses). Pursuant to a redundancy elimination mechanism inaccordance with the present invention, as acknowledgements arrive atcommunications processor 46, they are placed in a first-in/first-out(FIFO) acknowledgement reservation buffer 83, shown in FIG. 5, if thereis an upcoming acknowledgement reservation in the above-mentioned list.If, for some reason, there are no upcoming acknowledgement reservationsin the list, the acknowledgement packets are placed in a separate dataFIFO buffer 81 and treated as data packets for the purpose oftransmission. As an acknowledgement packet is about to be placed ineither the data FIFO buffer 81 or the acknowledgement FIFO 83, itscontents are examined to determine if the new acknowledgement containsmore current acknowledgement information than those currently bufferedand awaiting transmission. (It should be noted that an acknowledgementof a packet `p` implicitly acknowledges any previous packets `p-1`,`p-2`, `p-3`, etc.) If so, the acknowledgment contents are replaced withthe new information. Thus, acknowledgement traffic is kept to a minimumby eliminating redundant packets.

As each acknowledgement or data slot occurs, communications processor 46decides whether or not to transmit into the slot. If both theaforementioned acknowledgement reservation list and acknowledgement FIFOare not empty, the communications processor withholds all transmissionsuntil the acknowledgement slot occurs. At that time, the acknowledgementpacket is transmitted into the reserved acknowledgement slot and normalprocessing resumes. This procedure insures proper sequencing of data andacknowledgement packets.

It should be noted that PAD 48, upon transmitting a data packet to themaster station, typically will repetitively generate a poll packet forsome period of time until it receives an acknowledgement packet from themaster station. These repeated poll packets (acknowledgement packetswith a `poll` bit set) are a potential source of message trafficcongestion, but are still treated as normal message packets by thecommunications control unit 45. Consequently, the use of theacknowledgement redundancy feature serves to eliminate superfluoustransmissions over the return link channel.

As pointed out previously, in addition to the master stationpreassigning or reserving acknowledgement time slots on the return linkchannel for the transmission of acknowledgement messages from remotestations to the master station between data packet time slots, that arenormally accessed on a contention basis, provision is made for a remotestation to request preassignment or reservation of data packet timeslots by the master station, so that the requesting remote station willnot have to contend with other remote stations for the use of theremote-to-master channel to transmit its data, but, in a manner similarto the reservation of acknowledgement time slots, will have preassignedto it specific data packet time slots within which to transmit datapackets that are resident in its message queue 81.

More specifically, as pointed out briefly above, as message packets(data or acknowledgement) are supplied by terminal equipment serviced bythe remote station data communications equipment 33, the messages arequeued up in a FIFO 81. As the packets cycle through the FIFO and exitthe output buffer register, they are examined for communication controlindicators (tags) that may determine what type of communication controlaction will be taken.

If the volume of message traffic at a remote station builds up to aprescribed threshold level, which can be expected to cause the need forretransmission, resulting from the probable occurrence of collisionswith other contention slot access transmissions by other remotestations, then there is an increased likelihood that if the systemcontinues to operate in its normal contention data time slot mode, moreand more data packets will require transmission, so that eventuallythroughput from the remote station to the master station becomeseffectively nil.

To handle this overload condition, the present invention provides amechanism through which what are normally contention time slots for datatransmission are reserved or assigned for use by a remote station, sothat it is effectively guaranteed that a data packet currently bufferedat the remote station for transmission to the master station will haveaccess to an available time slot. For this purpose, communicationscontrol processor 46 employs a mechanism, to be described below, whichmonitors the contents of the outgoing message buffer 81 to determinewhether a number of saturation onset conditions have been satisfied thatmandate a request for the preassignment or reservation of data packettime slots by the master station for use by the remote station. If thenumber of entries within buffer 81 reaches a prescribed threshold (setin accordance with a preselected saturation/traffic density criteria)the data packets of the buffer are examined to determine whether anydata packet contains a retransmission flag that was set in the event ofa previously attempted transmission and reentry of the packet into thefirst or input stage of FIFO 81. If both the threshold andretransmission flag criteria have been satisfied, then the normalcontention mode of return link access by that remote station isinterrupted and a prescribed data time slot reservation request messageis transmitted to the master station. The control mechanism employed forrequesting reservation of data time slots may be best understood byreferring to the state diagram shown in FIG. 6.

Initially, during STATE 1, the contents of the outgoing message buffer81 are monitored to determine whether the number of entries in thebuffer (buffer level indicator BLI) exceeds a given threshold(BLI_(MAX)) and whether there is any data packet entry within the FIFO81 that has been tagged as a retransmission entry (namely, a data packetthat has been previously transmitted without the return of anacknowledgement from the master station, as indicated by a break in thePAD 48-supplied data packet sequence number). As long as the number ofentries or buffer level indicator BLI within the queue 81 is less thanthe threshold BLI_(MAX) and there are no pending retransmission datapacket requests, then the data packet contents of the output stage ofthe message buffer are transmitted in a normal contention mode andcontrol processor 46 forwards the data packets on to the burst modulator43 for transmission during the next data packet time slot.

If, however, both of the above conditions has been fulfilled (STATE 2),namely the size of the queue exceeds the threshold i.e. BLI>BLI_(MAX)and buffer 81 contains a data packet that has been tagged as aretransmission packet, the control mechanism proceeds to STATE 3 inwhich a prescribed reservation request is "attached" as part of theoverhead of the next data packet to be transmitted. Included as part ofthe information contained in the reservation request is the depth ofbuffer 81, namely BLI, in order to that the number of data time slotsreserved by the master station will be sufficient to empty out buffer 81and clear up the congestion problem.

When the data packet with the reservation request is transmitted in thenext contention time slot over the return link channel, a transmissionrequest soft-counter is incremented to indicate that a first request fora reservation assignment to the master station has been made. As long asthe contents of the counter is less than a prescribed value, and untilthe reservation request has been granted, the remote station willcontinue to retransmit its request for a reservation. If the request isimmediately granted and the remote station receives data time slotassignment message from the master station, it proceeds to STATE 4 andwaits for the reserved slots to occur. It then places the data packetsawaiting service in the assigned time slots, transmits the data to themaster station (STATE 5) and then returns to STATE 1. It should be notedthat every message transmitted over the outlink channel is received byall remote stations, although in normal circumstances only one(individually addressed) remote station will capture the packet. Thecontents of reservation assignment message, however, having had a globaladdress, will be read by all stations, so that their control processorswill comply with the reservation assignment and only that remote stationfor whom a reservation assignment has been awarded will us the assigneddata time slots. If the request is not immediately granted, then, aftera prescribed period of time, the remote station will proceed toincrement its reservation soft-counter (STATE 6) and retransmit thereservation request message (return to STATE 3). This procedure isrepeated for a specified number of retransmission intervals until areservation message is received or until the increment counter timesout. In the latter situation, the data slot reservation controlmechanism proceeds from STATE 6 to STATE 7, in which communicationcontrol unit 45 interrupts or suspends the forwarding of the reservationrequest message stored in buffer 82 to the burst modulator 43 and,instead, reverts to the normal contention mode, continuing to use thenext data message time slot that becomes available, until the currentcontents of the message queue have been serviced.

The effect of the data time slot reservation mechanism is to give remotestations having long or backed-up message queues the ability totemporarily empty their message buffers in a time slot-efficient manner(at the expense of delay). Namely, the queued messages are transmittedwithout contention, thus removing some of the load from the network. Asa consequence, the heavier the load on the network, the more theoperation tends to look like a time division multiplied access (TDMA)communication scheme rather than a slotted, demand assignment system. Itshould be recalled, however, that, although a TDMA scheme allows ahigher percentage of time slots to be used (there are no collisions),under normal circumstances it suffers a longer delay since, in effect, aTDMA system operates essentially as a polling mechanism.

As will be appreciated from the foregoing description, the improved linkutilization control mechanism according to the present inventionprovides a number of enhancements to demand assignment satellitecommunication networks that facilitate access by and throughput betweenusers of the network. By means of minor modification to a point-to-pointcommunications protocol (X.25), it is possible to simulatepoint-to-point communication ports and thereby readily interface what iseffectively a multidrop network with point-to-point landlinkcommunication resources.

In addition, the acknowledgement reservation mechanism substantiallyobviates the overhead and throughput penalties encountered in the use ofdata packets to transmit reduced size acknowledgement messages. Sincethe length of an acknowledgement message is considerably shorter thanthe length of a data packet, the dedication and reserving of suchacknowledgement time slots does not detrimentally impact channelutilization. Also, preassigning or reserving these reduced capacityslots for return-to-master acknowledgements serves to minimizecollisions and thereby improve overall network performance.

Finally, by augmenting the normal contention mode of operation with adata time slot reservation mechanism during periods of increased messageinput density at a remote station, transmission throughput isfacilitated so that congestion at the station is reduced.

While we have shown and described several embodiments in accordance withthe present invention, it is to be understood that the same is notlimited thereto but is susceptible to numerous changes and modificationsas known to a person skilled in the art, and I therefore do not wish tobe limited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed:
 1. For use with a communications system having a masterstation and a plurality of remote stations which communicate with oneanother over a communications channel, each of said remote stationshaving the capability of transmitting messages over said communicationschannel to said master station during time slots that are normallyaccessible by the remote stations on a contention basis, a method ofcontrolling the transmission of message, awaiting service in a messagestorage facility at a remote station, over said communication channel tosaid master station comprising the steps of:(a) in response to theoccurrence of a prescribed condition of the contents of said messagestorage facility in which messages awaiting transmission over saidcommunication channel to said master station are stored, transmitting arequest to said master station for the reservation of contention timeslots to be used by said remote station for the transmission of messagesawaiting service in its message storage facility; (b) in response toreceiving a message from said master station that contains theidentification of contention time slots that are to be reserved for useby said remote station, proceeding to transmit messages stored in saidmessage storage facility over said communications channel to said masterstation during said reserved time slots; and (c) in response to the lackof receipt of a message from said master station that contains theidentification of contention time slots that are to be reserved for useby said remote station, within a prescribed period of time aftertransmitting said reservation request, proceeding to transmit messagesstored in said message storage facility over said communications channelto said master station during non-reserved contention time slots.
 2. Amethod according to claim 1, wherein step (a) includes the step of, inthe absence of the occurrence of said prescribed condition, transmittingmessages, awaiting service in said message storage facility incontention time slots, over said communications channel to said masterstation, on a contention basis.
 3. A method according to claim 2,wherein said prescribed condition includes the condition in which thenumber of messages awaiting has reached a preselected number and thecondition in which said message storage facility contains a message thathas been previously transmitted and is awaiting retransmission by saidremote station to said master station.
 4. For use with a communicationssystem having a master station and a plurality of remote stations whichcommunicate with one another over a communications channel, each of saidremote stations having the capability of transmitting messages over saidcommunications channel to said master station during time slots that arenormally accessible by the remote stations on a contention basis, andarrangement for controlling the transmission of messages, awaitingservice in a message storage facility at a remote station, over saidcommunication channel to said master station comprising:first means,responsive to the occurrence of a prescribed condition of the contentsof said message storage facility in which messages awaiting transmissionover said communication channel to said master station are stored, forcausing a request message to be transmitted to said master station forthe reservation of contention time slots to be used by said remotestation for the transmission of messages awaiting service in its messagestorage facility; and second means, responsible to receipt of a messagefrom said master station that contains the identification of contentiontime slots that are to be reserved for use by said remote station, forcausing messages stored in said message storage facility to betransmitted over said communications channel to said master stationduring said reserved time slots, wherein said second means includesmeans, responsive to the lack of receipt of a message from said masterstation that contains the identification of contention time slots thatare to be reserved for use by said remote station, within a prescribedperiod of time after transmitting said reservation request, for causingmessages stored in said message storage facility to be transmitted oversaid communications channel to said master station during non-reservedcontention time slots.
 5. A method of controlling the transmission ofmessages, awaiting service in a message storage facility at a remotestation, over a communication channel to a master station in acommunications system having a master station and a plurality of remotestations, during time slots that are normally accessible by the remotestations on a contention basis, the method comprising the steps of:(a)transmitting a reservation request signal from a remote station to saidmaster station for the reservation of contention time slots to be usedby said remote station for the transmission of messages awaiting servicein its message storage facility as a function of saturation/trafficdensity; (b) in response to receiving a reservation acknowledgementsignal from said master station, proceeding to transmit messages storedin said message storage facility over said communications channel tosaid master station during reserved time slots; and (c) in response tothe lack of receipt of a reservation acknowledgement signal from saidmaster station within a prescribed period of time after transmittingsaid reservation request, proceeding to transmit messages stored in saidmessage storage facility over said communications channel to said masterstation during non-reserved contention time slots.
 6. A method accordingto claim 5, wherein step (a) includes the step of, in the absence of theoccurrence of a prescribed condition that is set in accordance with saidsaturation/traffic density, transmitting messages awaiting service insaid message storage facility over said communications channel to saidmaster station in contention time slots on a contention basis.
 7. Amethod according to claim 6, wherein said prescribed condition includesthe condition in which the number of messages awaiting has reached apreselected number and the condition in which said message storagefacility contains a message that has been previously transmitted and isawaiting retransmission by said remote station to said master station.8. A device for controlling the transmission of messages, awaitingservice in a message storage facility at a remote station, over acommunication channel to a master station in a communications systemhaving a master station and a plurality of remote stations, during timeslots that are normally accessible by the remote stations on acontention basis, comprising:means for transmitting a reservationrequest signal from a remote station to said master station for thereservation of contention time slots to be used by said remote stationfor the transmission of messages awaiting service in its message storagefacility as a function of saturation/traffic density; means forresponding to the receipt of a reservation acknowledgement signal fromsaid master station, and transmitting messages stored in said messagestorage facility over said communications channel to said master stationduring reserved time slots; and means, responsive to the lack of receiptof a reservation acknowledgement signal from said master station withina prescribed period of time after transmitting said reservation request,for transmitting messages stored in said message storage facility oversaid communications channel to said master station during non-reservedcontention time slots.
 9. A device for controlling the transmission ofmessages, awaiting service in remote station, over a communicationchannel to a master station in a communications system having a masterstation and a plurality of remote stations, during time slots that arenormally accessible by the remote stations on a contention basis,comprising;a storage unit for storing messages awaiting service; meansfor attaching a retransmission flag to a message that indicates anunsuccessful transmission attempt of said message has previously beenmade; means for comparing the number of messages stored in said storageunit against a prescribed threshold; means for determining whether anymessage stored in said storage unit has an attached retransmission flag;and means for retransmitting a reservation request signal from a remotestation to said master station for the reservation of contention timeslots to be used by said remote station for the transmission of messagesawaiting service in its storage unit when the number of messages storedin said buffer reach the prescribed threshold and at least one of themessages stoed in said storage unit has an attached retransmission flag.10. The device of claim 9, further comprising a counter which iscontrollably incremented after the reservation request signal has beenfirst transmitted.
 11. The device of claim 10, further comprising meansfor receiving a reservation acknowledgement signal from the masterstation indicating contention time slots are reserved for thetransmission of messages stored in said storage unit of said remotestation transmitting the reservation request signal.
 12. The device ofclaim 11, further comprising means for re-transmitting the reservationrequest signal until said means for receiving receives a reservationacknowledgement signal or until said counter times out, and means forreverting, in response to a time out by said counter to a normalcontention mode in which said messages are sent to said master stationduring time slots accessible to said remote stations on a contentionbasis.
 13. The device of claim 12, wherein said storage unit comprises afirst in, first out (FIFO) buffer.
 14. A method of controlling thetransmission of messages, awaiting service in a remote station, over acommunication channel to a master station in a communications systemhaving a master station and a plurality of remote stations, during timeslots that are normally accessible by the remote stations on acontention basis, comprising the steps of:(a) storing messages awaitingservice; (b) attaching a retransmission flag to a message that indicatesthat an unsuccessful transmission attempt of said message has previouslybeen made; (c) comparing the number of messages stored in step (a)against a prescribed threshold; (d) determining whether any messagestored in step (a) has an attached retransmission flag; and (e)transmitting a reservation request signal from a remote station to saidmaster station for the reservation of contention time slots to be usedby said remote station for the transmission of stored messages awaitingservice when the number of stored messages reach the prescribedthreshold and at least one of the stored messages has an attachedretransmission flag.
 15. The method of claim 14, further comprising thestep of (f) incrementing a counter after the reservation request signalhas been first transmitted.
 16. The method of claim 15, furthercomprising the step of (g) receiving a reservation acknowledgementsignal from the master station indicating contention time slots arereserved for the transmission of stored by messages, said remote stationtransmitting the reservation request signal.
 17. The method of claim 16,further comprising the step of (h) re-transmitting the reservationrequest signal until a reservation acknowledgement signal is received bythe remote station transmitting said reservation request signal or untilthe counter incremented in step (f) times out, and reverting in responseto a time out of said counter, to a normal contention mode in which saidmessages are sent to said master station during time slots accessible tosaid remote stations on a contention basis.
 18. The method of claim 17,wherein step (a) comprises storing messages in a first in, first out(FIFO) buffer.